Measuring devices of this kind are already known which include one or more sensors which are arranged to measure various physical and/or chemical parameters in an air stream. The most widely used type of measuring device comprises a temperature sensor or temperature sonde, in the form of a thermistor. This sensor is located in a selected location in the vehicle, in the path of the air stream before the latter is introduced into the cabin of the vehicle.
A conventional motor vehicle typically has a heating and ventilating (and/or air conditioning) installation which includes, in particular, an air inlet casing which has a fresh air inlet, for introduction of fresh air from outside the vehicle, controlled by a flap valve, together with another air inlet for admitting air recirculated from the cabin of the vehicle and controlled by another flap valve. When such a vehicle is provided with a temperature sensor as mentioned above, the latter is often placed in the air inlet casing of the installation, upstream of the flap valve that controls the admission of fresh air from outside the vehicle (i.e. upstream with reference to the direction of flow of the air stream).
Such an arrangement does have certain disadvantages. In this connection, if the sensor is operating correctly when the installation is in its fresh air mode, then since the sensor is under those conditions ventilated by the stream of fresh air, the same is not true in the alternative situation where the installation is operating in its recirculated air mode. In this latter case, the sensor is not in fact ventilated, and the information derived from the sensor is corrupted by the thermal radiation of the vehicle itself, and in particular that coming from the engine, since the engine is generally quite close to the heating, ventilating or air conditioning installation itself.
In order to overcome this drawback, it has been proposed that the latest measured value of the parameter concerned, represented by the signal from the sensor, should be memorised before the installation is changed from its fresh air mode to its recirculated air mode. It has also been proposed to mount the temperature sensor on the flap valve which controls the admission of fresh air, so as to make its position variable and to improve its ventilation.
However, these solutions do involve certain complications in the construction of the installation; yet, at the same time, they still fail to ensure that the temperature sensor will always give signals that represent the true values of the parameter to be measured.
Apart from temperature sensors, the sensors may of course include at least one other sensor for measuring at least one other parameter. These may for example comprise suitable sensors for measurement of humidity (absolute or relative) in the air stream, with a view to controlling the humidity within the cabin at a required level. Since these humidity sensors are generally located in the same position as the temperature sensors, they are subject to the same drawbacks as have been mentioned above.
In addition, chemical, or physico-chemical, sensors may be provided for dealing with pollution problems. These sensors are of a type which are capable of detecting the presence of noxious gases, especially carbon monoxide, oxides of nitrogen, and hydrocarbon exhaust gases. However, pollution sensors at present pose a double problem. First of all, since these sensors are capable of giving a command for passing air in a recirculating mode, they must also be capable of giving the opposite command when the pollution has been dispersed. However, this is not possible if the sensors are not correctly ventilated or aerated by the air stream. In addition, the pollution sensors are in general heated to temperatures which may attain a local mean value of the order of 100 to 400 degrees C., in order to activate the chemical reactions which are the basis of the operation of such sensors. Here again, if the sensors are not correctly ventilated, they may give erroneous values, as may the adjacent sensors. Another undesirable result that may occur is deterioration of the sensors under the effect of high temperatures.